Accelerated atherosclerosis displays a complex pathogenesis including alterations in lipids, inflammatory state involving the immune system. HDL apoA-I protects against these changes mainly through its ability to organize and recruit cholesterol and oxygenated forms of cholesterol and phospholipids from immune cells protecting them from dysregulation and apoptosis. In the current proposal, we will investigate the molecular mechanisms responsible for immune cell cholesterol deposition, accelerated atherosclerosis and the development of an autoimmune phenotype in response to an atherogenic diet in LDL receptor, apoA-Idouble knockout (DKO) mice. In previous studies, when DKO and LDLr-/- (SKO) mice were fed an atherogenic diet, DKO mice developed enlarged peripheral lymph nodes (LNs) and spleens compared to SKO mice. DKO LN were enriched in cholesterol ester (CE) and contained expanded populations of CE enriched T, B, dendritic cells and macrophages. Plasma antibodies to dsDNA and oxidized LDL were also increased in DKO suggesting an autoimmune phenotype. Both LN enlargement and LN CE accumulation were prevented when diet-fed DKO mice were treated with apoA-I at the time the diet was initiated. Regardless of the level of dietary cholesterol, DKO mice consistently showed lower plasma cholesterol than SKO mice, yet greater aortic cholesterol deposition and inflammation. Therefore, the goal of this proposal is to use the DKO mouse to investigate the mechanisms by which apoA-I 1) modulates CE and oxysterol accumulation and activation in lymphocytes, 2) alters the proliferation and/or apoptosis of CE loaded lymphocytes, 3) affects the contribution of T cells and DC to plaque infiltration in both progression and regression of atherosclerosis in diet-fed DKO mice.